When a circular saw operates at the high rate of rotation necessary to cut wood, the outer edges of the blade have a tendency to deviate from the cutting line, either to one side or the other, or to both sides, producing a wavy cut. There is also a tendency for the saw edge to start its cut to one side or the other of the intended line, whereupon the blade tends to lead the cut in that direction. Paradoxically, when the blade is stopped, it is found to be straight and true.
The uneven cutting above described is a result of invisible defects in the saw blade, which defects include both minute surface deformities and improper distribution of residual tension stresses in the blade. The centrifugal force which results from high-blade rotational speeds induces substantial tension stress in the blade. This tension is superimposed on any residual stresses in the blade, such that the resulting non-uniformity of tension produces uneven strain. Any irregularities in the blade surface also interfere with true running, as the tension changes when the blade approaches its cutting speed. Thus, a blade may be deflected from a true course by either of these types of defects.
Areas on the surface of a blade wherein residual tension stresses vary significantly from the normal tension level in that portion of the blade are commonly referred to as "tight" or "loose" spots. Surface deformities as above noted include convex and concave deviations from a flat plane, including bulges, ridges, kinks, etc. Consequently, a critical part of the maintenance of circular saws is removing these dents or bends, i.e., leveling, as well as tuning the saw plate to operate at typical rotational speeds by stretching the plate in specifically determined locations to cause tension at the periphery of the saw plate, i.e., tensioning.
Leveling and tensioning operations have typically been done manually using various hammers, anvils and gauges. Indeed, several equipment manufacturers build "stretcher rolls" which assist in such manual tensioning and leveling of saw blades.
Manual tensioning and leveling is a highly skilled craft requiring intuitive analyses of problems and the development of unique solutions. Workers who do this are known as sawsmiths. They are among the highest paid of all shop trades. Because of the great skill required, the number of sawsmiths practicing this trade has remained greatly limited and the craft has often been practiced in secret. Besides the hammers, anvils, straight edges and other tools, the sawsmith must utilize his senses. These must be trained by long experience such that he can inspect a saw blade for imperfections, mark the blade as to type and location of imperfections, and then employ the proper hammering patterns and tools to remove the imperfections from the saw. Proper hammering over both surfaces produces a more uniform tension distribution, as well as removing surface flaws, thereby to enable the saw to run straight and true at cutting speeds.
In order for a sawsmith properly to hammer a saw blade, generally he first bends it around an axis parallel to one of its diameters so that its surface takes the form of a curved cylindrical segment. This is called "dishing" the blade. The sawsmith then applies a straight edge to the blade surfaces at right angles to their curvature. The blade reveals to his practiced eye the nature and location of any defects. The sawsmith then marks each defect such that he may thereafter select those tools and techniques best suited for its correction. Typically, the sawsmith places the saw on an anvil and commences hammering the marked areas, guided only by his marks. Of course, he must be extremely careful to hammer in the proper manner and in the correct location. While performing this rectifying operation, the sawsmith may also hammer generally over the entire surface of the saw blade, thereby to obtain an overall distribution of tension appropriate to the particular blade involved.
When he applies his straight edge as above described, the sawsmith actually is locating bulges and depressions representing either physical distortions or "tight" or "loose" spots on the surface of the blade. Tight spots on the blade surface tend to bulge inwardly from the concave surface when the saw is bent or dished. Similarly, loose spots tend to bulge outwardly. When the blade is laid flat, such areas generally cannot be detected. Kinks, ridges and other physical distortions in the blade can also be detected by the way they stand up under a straight edge. Additionally, the manner in which the saw blade actually curves when it is bent or dished is, to the skilled hand and eye of the sawsmith, a measure of the overall tension distribution in the saw. As can be readily understood, proper leveling and tensioning of a saw blade is a highly skilled art, the cost of which is quite significant.
In view of the cost inherent in proper manual hammering of a saw, it was natural that the trade would attempt to create a machine which could accomplish such hammering at lesser cost. U.S. Pat. No. 510,210 was an early attempt at achieving a device which could be used to straighten circular saws. The apparatus therein disclosed provided rollers or blocks on both sides of the saw blade which were so disposed in relation to one another that the blade could be deformed in a direction opposite to that of the distortion.
U.S. Pat. No. 1,053,746 disclosed a similar machine utilizing rollers which moved toward and away from each other. The rollers were arranged to take a circular course over the surface of the saw, or, alternatively, a spiral course, in which case they could automatically feed the saw. Transverse movements of the saw-supporting structure were themselves monitored by a hand lever. U.S. Pat. No. 1,096,756 disclosed an improvement to the '746 patent and, in particular, to the saw-supporting and saw-controlling structure thereof.
U.S. Pat. No. 1,290,491 disclosed another circular saw stretching and tensioning machine wherein a pair of straightening rollers were each coaxially mounted on shafts positioned parallel to the sides of the saw blade.
U.S. Pat. No. 3,964,348 disclosed a circular saw blade hammering machine utilizing a proximity detector and wherein the blade was dished to locate tight and loose portions. A hammer was utilized to strike the blade wherever a high spot was detected. The action of the hammer was meant to replicate that of the sawsmith during his manual operations.
U.S. Pat. No. 4,852,430 disclosed a band saw blade straightening machine with sensor means for scanning the surface of the blade and generating a signal responsive to unevenness therein. A straightening station with two pairs of straightening rollers was provided. The straightening rollers comprised two pairs of rollers. One pair had a roller disposed above the band saw with a convex roller surface for smoothing upwardly directed unevenness, and a roller with a concave rolling surface disposed opposite underneath the band saw. The other pair for smoothing downwardly directed unevenness had a roller disposed above the band saw with a concave rolling surface and a roller with a convex rolling surface disposed underneath. The machine, however, was unsuitable for circular saw blades because it is not possible to define straight pathways with straightening rollers and a sensor behind each on a circular saw blade.
U.S. Pat. No. 4,875,393 disclosed an apparatus having a pair of top rolls and a pair of bottom rolls with one of the top rolls and one of the bottom rolls in each pair being of larger diameter than the other of the pair in both top and bottom positions. By providing a pair of rolls at the top and a second pair at the bottom with one of the two rolls in each pair being larger in diameter than the other, better leverage was allegedly possible to distort a saw blade either up or down.
U.S. Pat. No. 5,269,205 disclosed still another saw blade straightening machine wherein the blade was scanned with an electronic indicator mounted with a fixed reference from a beam. This scanner or sensor is unable to supply proper usable information to the computer which governs the action of the pressing rollers. Specifically, referencing a sensor from a fixed beam fails to recognize that a typical defect in a saw blade is often actually the result of three bends.
For example, a bulge in an upper surface of a saw blade is often the result of two concave deformations, which cause a convex deformation therebetween. The sensor disclosed in the '205 patent is unable to detect the fact that, in many such cases, only the concave areas are the actual defective areas, the apparent bulge being in fact a convex area, which is the natural result of the concave areas on either side. A sensor referenced from a fixed beam can only find and attempt to correct the convex area between the two concave areas.
Furthermore, the sensor disclosed in the '205 patent must be calibrated for each plate thickness. Also, the sensor disclosed has a very tiny contact point which is vulnerable to wear-related errors. Also, a properly tensioned saw plate may sag at its periphery when positioned horizontally and supported only at its center eye. The '205 patent sensor detects this condition as a very long dent down and, consequently, relays erroneous information to the computer.
Additionally, the '205 patent machine requires two complete sets of rollers, one set and all its supporting assemblies to push a convex deformation downwardly, and an additional set of rollers with an additional set of supporting assemblies to push a concave deformation upwardly.